Synopsis C-FCGE, a Beechcraft King Air A90, was on a training flight in level cruise at 11 500 feet above sea level (asl) in the vicinity of North Bay, Ontario, when the crew experienced a vibration in their aircraft and an uncommanded turn to the right. The decision was made to divert to NorthBay. During the descent, a severe vibration developed and control of the aircraft was briefly lost. After this severe vibration ceased and control was regained the aircraft required significant left rudder to maintain co-ordinated flight. An uneventful landing was completed. No injuries occurred. Other Factual Information During the descent, at approximately 8 500 feet asl, a severe vibration developed which violently shook the aircraft and rapidly moved the rudder and ailerons causing the hands and feet of the pilot flying to be displaced from the control wheel and rudder pedals. After approximately 30 to 45 seconds, the vibration stopped and control was regained. The descent was then continued at 140 knots, as the aircraft seemed controllable at this speed. However, to maintain coordinated flight, the crew had to hold approximately 2/3 left rudder. The remainder of the descent, approach and landing were uneventful and the aircraft landed safely. No injuries occurred. A maintenance recovery team was dispatched and examined the aircraft. They determined that the hardware connecting the rudder trim actuator push/pull rod to the rudder trim horn was missing. After replacement of the missing hardware and a free play check, the aircraft was ferried to Ottawa. The aircraft was returned to service after a severe turbulence check and non-destructive testing (NDT) of fittings in the empennage and wings. The rudder trim is set by turning a wheel in the cockpit. This movement is converted into a fore and aft motion of the push/pull rod. The rod, attached by a clevis and bolt to a horn on the starboard side of the rudder trim tab, displaces the tab either to the left or right. This causes the rudder to be moved in the opposite direction thereby trimming out the control forces necessary to maintain co-ordinated flight. (See Figure 1) The hardware attaching the push/pull actuating rod to the rudder trim horn was last replaced when the aircraft was painted during the period 25 January 1995 to 10 March 1995. Since then, the last maintenance completed in this area was the rudder trim tab free play check on 24January 1997. Part of this check calls for a visual inspection of the trim tab actuating system and directs that any inconsistencies be rectified prior to the free play check. No problems were detected and the component passed the check. As a result, the attaching hardware was not disturbed during the process. The aircraft flew a total of 59 hours between the free play check and the occurrence. The Beechcraft 90, A90, and B90 Series Illustrated Parts Catalogue (IPC Fig 54, page 3, Index #42) specifies that the attachment hardware used to connect the rudder trim actuator include an AN173 bolt (no length specified), two AN320-3 castellated nuts and two AN380-2-2 cotter pins. The aircraft operator also uses Beechcraft 100 aircraft in a similar role. The Beechcraft 90 and 100 are alike in many respects including the rudder trim tab and actuator. The IPC for the Beechcraft 100 specifies an AN173-5 bolt in this assembly along with one AN960-10L washer, one AN320-3 castellated nut and one MS24665-132 cotter pin (IPC 27-21-01, page 3, Index #3). Inspection of other Beechcraft A90 aircraft used by the operator revealed that the rudder trim actuator is typically attached to the horn using an AN173-5 bolt, washers of varying thickness and number (to ensure the correct orientation of the cotter pin hole in the bottom of the bolt with the indentations in the nut), one AN320-3 castellated nut and one cotter pin.